In order to have a comfortable environment and sufficient air pressure to breathe at the altitudes at which jet airliners fly, a supply of warm compressed air is required.

This is nowadays (with the sole exception of the new Boeing 787) supplied direct from the jet engines and is known as “bleed air“. It is mixed inside the aircraft with recirculated cabin air at a ratio of 50/50. Although some of the air is subsequently recirculated, all of the air originates from the jet engines.

Bleed air comes from the compressor section of the jet engine, which has to be lubricated. Jet engines mostly have “wet seals” to keep the oil and air apart, which cannot be 100% effective. Furthermore these seals, like any mechanical component, slowly wear out and their effectiveness gradually declines, especially when the engine is working hard, such as climbing under full throttle. They may also fail suddenly and will then let a significant amount of oil into the very hot compressed bleed air, resulting in fumes and/or smoke entering the cabin. This is known as a “fume event”.

There are no filters in the bleed air supply to stop this happening.

Note that the oil used to lubricate jet engines is not based on petroleum hydrocarbons as are lubricants for internal combustion engines used in motor cars, outboard motors, tractors etc. Jet engines operate at much higher temperatures and, therefore, use special synthetic chemicals as oil. They also contain organophosphate additives as antiwear agents and other aromatic hydrocarbons as antioxidants. Some of the oil gets partially decomposed, i.e. chemically altered (“pyrolysed”) due to the high temperatures in the engine.

In summary, the contamination is composed of the “oil”, the additives, and the decomposition products. The toxicity seems to be due to the last two of these three.

How often does a “fume event” occur?

The UK Committee on Toxicity of Chemicals in Food, Consumer Products and the Environment (COT) accepts that fume events occur on 1 flight in 100 in its 2007

However, on some aircraft types crews report that they experience fumes to some degree on every flight and as the definition of “fume event” is not agreed upon, it makes it impossible to

give a true figure.

How can I tell if the cabin air is contaminated?

Slight leakage of oil into the cabin may be detected by smell (descriptions such as ‘sweaty socks’, ‘wet dog’, ‘vomit’, ‘sweet oily smell’ have been used). Background levels of contamination may not be detectable by smell. There are no chemical sensors in modern jet aircraft. If a “fume event” occurs bluish haze or smoke in the cabin may be visible.

How prevalent is “normal” contamination?

By “normal” we mean slight (relative to a fume event) but significant contamination. The degree of contamination depends on jet engine type and how recently it was serviced, among other factors. There are few reliable measurements but based on what has been done we estimate that about a quarter of flights suffer slight but significant contamination. It is important to remember that this contamination might be continuous throughout a flight; hence the total exposure might end up as much as after a brief fume event.

Fume events also deposit substantial residues on all the interior surfaces of the cabin. These residues then slowly desorb, adding to what is coming directly from the engines.

What are the symptoms?

Symptoms may be acute, i.e. for a short time or chronic, i.e. long-lasting.

Any combination of the following may be experienced:

Fatigue – feeling exhausted, even after sleep

Blurred or tunnel vision

Shaking and tremors

Loss of balance and vertigo

Seizures

Loss of consciousness

Memory impairment

Headache

Tinnitus

Light-headedness, dizziness

Confusion / cognitive problems

Feeling intoxicated

Nausea

Diarrhoea

Vomiting

Coughs

Breathing difficulties (shortness of breath)

Tightness in chest

Respiratory failure requiring oxygen

Increased heart rate and palpitations

Irritation of eyes, nose and upper airways.

which is why the term “syndrome” is used. Many general medical practitioners are unaware of the condition and may diagnose sufferers with illnesses such as psychological or psychosomatic disorders (i.e., they’ll tell you “it’s all in your mind”), Chronic Fatigue Syndrome (CFS), “mysterious” viral infections, sleep disorders, depression, stress or anxiety – or simply “jet lag”, which is caused by crossing time zones (and does weaken the immune system).

Although some of these disorders may form part of Aerotoxic Syndrome, such part-diagnoses on their own miss the root cause of the problem, which is inhaling toxic oil components and derivatives. Furthermore, any misdiagnosis is likely to lead to inappropriate treatments, which may even make the condition worse.

Aviation medicine specialists are aware of the problem but Aerotoxic Syndrome does not seem to have gained “official” acceptance among the majority of them. Hence, despite (or because of) their expert knowledge they are likely to seek for other explanations – and there are plenty of neurological symptoms associated with aviation that have nothing to do with inhaling oil.

Is Aerotoxic Syndrome treatable?

For short exposures, the effects are usually reversible and will resolve themselves. But serious or repeated low dose exposures can lead to severe symptoms. Permanent neurological damage may be caused, which can not be recovered from.

There are many prematurely medically retired aircrew with “mysterious” neurological symptoms, most have been grossly misdiagnosed.

There is no magic cure, but there are specialists who may be able to help and strategies to aid recovery.

The first step is to recognise the problem, and avoid or limit further exposures.

Can anyone be affected?

Yes. The toxins attack the central nervous system (including the brain). It’s not easy to predict how different exposures may affect different people, due to the genetic variability of individuals. Hence, one person’s body may have less success than another’s at detoxifying contaminants and so be affected after just one flight, whilst others may be unaffected after years of exposure. Depending on detoxifying efficiency, the adverse health effects may be cumulative. Therefore, anyone frequently (which means once or more a week) repeatedly exposed is especially at risk.

Even after a serious fume event, it is possible that no symptoms show initially, but a few days later ill health may kick in.

Airlines provide no protection to passengers against fume events. The drop-down masks available for use following loss of cabin pressure are designed to supplement oxygen and are not effective for removing contaminants.

Some aircraft have a worse history with the worst offenders being the BAe 146, Boeing 757 and Airbus 319.

How long has this problem been known about?

The first well-documented case was of a C-130 Hercules navigator becoming incapacitated after breathing contaminated cabin air in 1977. The neurotoxic properties of organophosphates have been known about since before the Second World War. The toxicity of heated jet oil was demonstrated in the 1950s.

Are there solutions?

In today’s existing modern bleed air aircraft, the quality of cabin air could be improved, and the risk of contamination by engine oil reduced, with these solutions:

The new Boeing 787 Dreamliner is the obvious answer as it eliminates the possibility of cabin air contamination. Instead of bleed air, cabin air is supplied by electrically-driven compressors taking their air directly from the atmosphere.

As bleed air is not presently filtered, installation of bleed air filtration systems could eliminate the problem, although a technically efficient system does not yet seem to have been developed.

A less toxic oil formulation could lead to significant improvement. The French oil company NYCO is continuously developing such oils.

Chemical sensors to detect contaminated air in the bleed air supplies – instead of human noses – would alert pilots to problems, allowing prompt preventive action.

Is the aviation industry addressing the issue?

Numerous independent scientific studies have produced clear evidence of contaminated cabin air being the cause of chronic health problems. On the other hand, various governments and regulatory authorities have commissioned research, which, while admitting association (between contaminated cabin air and chronic health problems) have typically stopped short of admitting causation. The aviation industry has tended to use the latter set of research (despite its often dubious scientific quality) to deny the existence of the problem, while ignoring the evidence of the independent studies.

One feature of the complex international situation of aviation is that the regulating authorities, while nominally government agencies, are financed and controlled by the industry and therefore follow the industry’s desires.

Doubtless mindful of the apparent expense of addressing the issue, industry seems to have chosen to ignore or deny the problem. Until now this has been a sustainable strategy because few aircrew, doctors or passengers are aware of the existence of aerotoxicity.